1. Field of the Invention
This invention relates to a male screw and more particularly to a male screw that can prevent loosening thereof and a method for manufacturing such male screw.
2. Prior art
As a method for preventing a screw from coming loose, the "double nut" structure, wherein two nuts are screwed onto a bolt in contact with each other, has been well known. In this "double nut" structure, strong forces pushing each nut are caused between the nuts to act as a tensile force on the bolt via the threads of the nuts and the bolt, and thereby the threads of the nuts and the thread of the bolt are pressed against each other to prevent the nuts and the bolt from coming loose.
The "double nut" structure, however, cannot be applied to a screw which forms its own internal thread in a object to be screwed in, such as a self tapping screw, wood screw or the like, since it is a method for preventing the loosening of a screw at the side of the nuts. Also, it poses a problem that it costs much labor since two nuts should be screwed on a bolt.
Besides the "double nut" structure, there have been known many methods for preventing a screw from coming loose, but most of them prevent loosening of a screw by using an extra component such as a special washer or the like or by applying adhesive to the screw. Such methods, however, have a disadvantage that the operation of mounting the extra component on the screw or applying adhesive to the screw costs much labor, decreases operating efficiency and increases operation cost.